Fixes#23223.
## Why
Malformed AGENTS instructions should not fail silently. The reported
issue had invalid UTF-8 in a global `AGENTS.md`; before this change,
Codex treated that decode failure like a missing file, so the personal
instructions disappeared without a user-visible explanation and the
rollout had no `# AGENTS.md instructions` block.
Project-level AGENTS files already used lossy decoding, so their
instructions still appeared, but invalid bytes were replaced without
telling the user. Global and project AGENTS files should behave
consistently: keep usable instruction text when possible, and surface a
diagnostic when bytes had to be replaced.
## What changed
Global `AGENTS.override.md` and `AGENTS.md` loading now reads bytes and
decodes with replacement characters on invalid UTF-8, matching
project-level AGENTS behavior. Both global and project AGENTS loading
now emit a startup warning when invalid UTF-8 is found, and both keep
the instruction text with invalid byte sequences replaced.
Missing files, non-file candidates, empty files, and the existing
`AGENTS.override.md` before `AGENTS.md` precedence keep their current
behavior.
## How users see it
The warnings flow through the existing startup warning surface.
App-server clients receive config-time startup warnings as
`configWarning` notifications during initialization, and thread startup
emits startup warnings as thread-scoped `warning` notifications.
Global AGENTS invalid UTF-8 warnings can appear on both surfaces.
Project-level AGENTS invalid UTF-8 warnings are discovered while
building thread instructions, so they appear as thread-scoped `warning`
notifications. Clients that render warning notifications in the
conversation surface show the message as a visible diagnostic instead of
silently hiding or altering instructions.
## Summary
`codex.emitImage` accepted arbitrary image MIME types for byte payloads
and data URLs. That allowed a value like `image/rgba` to be wrapped as
an `input_image`, even though it is not a supported encoded image
format, so the invalid image could reach the model-input path and
trigger output sanitization.
This results in a panic in debug builds because the output sanitization
is meant as a final safety net, not a primary means of rejecting invalid
image types. I've hit this case multiple times when executing certain
long-running tasks.
This PR rejects unsupported image MIME types before they are emitted
from `js_repl`.
## Changes
- Validate `codex.emitImage({ bytes, mimeType })` in the JS kernel so
only encoded PNG, JPEG, WebP, or GIF payloads are accepted.
- Apply the same MIME allowlist to direct image data URLs, including the
Rust host-side validation path.
- Clarify the JS REPL instructions so agents know byte payloads must
already be encoded as PNG/JPEG/WebP/GIF.
Eric TrautandGitHub
Curtis 'Fjord' HawthorneandGitHub
pakrym-oaiandGitHub